This invention relates to the field of two-way communication systems including repeaters, and particularly to the prevention of interference caused by two repeaters hearing the same portable unit.
Many two-way communication systems are known which include a base station and a multiplicity of vehicles, each vehicle containing a mobile transceiver, a repeater and a portable unit. Typically, the mobile transceiver communicates with the base station on one or two frequencies. When the portable unit is removed from the vehicle, as at the scene of a police investigation, the repeater in that car is activated to communicate with the portable on another frequency, the repeater serving as a link between the portable and the transceiver in the vehicle. When it is desired to transmit back to the base station a coded tone signal (CTCSS) is transmitted to the repeater unit. For portable-to-portable transmission at the location, the tone is not transmitted to the repeater, and the repeater does not couple the received message to the mobile transceiver for transmission to the base.
Prioritizing schemes have been developed whereby only one repeater at one location will communicate with all portables at that location. The presence or absence of the coded tone is utilized in some prioritizing schemes. Two such schemes are disclosed in U.S. Pat. Nos. 4,056,779 and 4,056,780, both assigned to the same assignee as is the present invention.
It may, however, happen in a densely populated area that two such investigations may be in progress in close proximity at the same time, and there would normally be two repeater units having priority status. In such a case, it is possible for a man carrying a portable unit to be within the receiving and squelch range of both priority repeaters although the repeaters are out of range of each other. When he transmits a message intended for his own priority repeater, each priority repeater receives his message and couples it to the associated mobile transceiver for transmission back to the base. This, of course, causes interference and, conceivably, loss of vital information. It would be advantageous to be able to reduce the receiving range of a repeater for portable-to-base transmissions only, while maintaining a more extended range for repeater transmissions, since the latter is necessary in order to establish a priority vehicle at that location.
Systems with dual squelch controls are well known, using a manually operated switch to enable either a standard carrier/noise controlled circuit or a squelch circuit which only opens when a coded tone signal is received. In the first mode, all signals of the proper frequency will be received, while in the latter mode, only those messages intended for that unit will be heard. Other dual squelch systems detect both carrier and coded tone simultaneously at all times.